1. Field of the Invention
The invention relates generally to the field of marine seismic data acquisition and processing. More particularly, the invention relates to methods for processing marine seismic signals to attenuate the effects of certain types of noise.
2. Background Art
Seismic surveying is known in the art for determining structures of rock formations below the earth's surface. Seismic surveying generally includes deploying an array of seismic sensors at the surface of the earth in a selected pattern, and selectively actuating a seismic energy source positioned near the seismic sensors. The energy source may be an explosive, a vibrator, or in the case of seismic surveying performed in a body of water such as the ocean, one or more air guns or water guns.
Seismic energy which emanates from the source travels through the earth formations until it reaches an acoustic impedance boundary in the formations. Acoustic impedance boundaries typically occur where the composition and/or mechanical properties of the earth formation change. Such boundaries are typically referred to as “bed boundaries.” At a bed boundary, some of the seismic energy is reflected back toward the earth's surface. The reflected energy may be detected by one or more of the seismic sensors deployed on the surface. Seismic signal processing known in the art has as one of a number of objectives the determination of the depths and geographic locations of bed boundaries below the earth's surface. The depth and location of the bed boundaries is inferred from the travel time of the seismic energy to the bed boundaries and back to the sensors at the surface.
Seismic surveying is performed in the ocean and other bodies of water (“marine seismic surveying”) to determine the structure and composition of rock formations below the sea bed. Marine seismic surveying systems known in the art include a vessel which tows one or more seismic energy sources, and the same or a different vessel which tows one or more “streamers.” Streamers are arrays of seismic sensors in a cable that is towed by the vessel. Typically, a seismic vessel will tow a plurality of such streamers arranged to be separated by a selected lateral distance from each other, in a pattern selected to enable relatively complete determination of geologic structures in three dimensions. It is also known in the art to place cables having seismic sensors (“ocean bottom cables”) along the sea bed, and actuate a seismic energy source in the water. Typically, the seismic energy source will be towed by a vessel just as in streamer-type surveying.
Streamers typically include a plurality of pressure responsive sensors such as hydrophones disposed at spaced apart locations along each streamer. Streamers have been developed more recently that include both pressure responsive sensors and particle motion responsive sensors such as geophones. By using both pressure responsive and motion responsive sensors, it may be possible to obtain seismic information within a frequency range that is not well illuminated when pressure responsive sensors are used alone. Such frequency range results from reflection of the seismic energy from the water surface, and is referred to as the “ghost notch” in the seismic frequency response. An example of a seismic streamer using both pressure responsive sensors and motion responsive sensors is disclosed in U.S. Pat. No. 7,239,577 issued to Tenghamn et al. and assigned to the assignee of the present invention.
It has been determined by testing and use of streamers having both pressure responsive and motion responsive sensors that the signals detected by the motion responsive sensors are particularly noisy at low frequencies (approximately 0 to 20 Hz). Such noise is believed to be related to movement of the streamers in the water. The foregoing noise issue has been addressed by methods such as one disclosed in U.S. Pat. No. 7,359,283 issued to Vaage et al. and assigned to the assignee of the present invention. The upper limit of the noise frequency range depends on many factors. One of them is vessel speed: the faster the vessel, the higher the noise level, and therefore the higher the upper limit.
The first notch frequency fn of the hydrophone ghost is related to the depth d of the sensor by the formula:fn=V/2d Where V is the velocity of sound in water. For the dual-sensor summation to work properly, the geophone data must have signal to fill the hydrophone frequency notches. Thus, the upper limit of the geophone noise frequency range must be smaller than fn. Therefore, there is a trade-off between the depth in the water at which the streamers may be towed and the speed at which towing may take place. Increased towing speed generates more motion-induced noise, which increases the upper limit of the noise frequency range and consequently reduces the depth at which the streamers can be towed.
What is needed is a method for automatically determining the lowest recoverable frequency from the measured seismic signals that can be performed on a seismic vessel as a real time data quality control measurement so as to ensure optimized towing depth and vessel speed.